In recent years, the realization of a large size of a TFT matrix panel in which TFTs are formed on an insulating substrate and the realization of a high driving speed have rapidly been being progressed. A manufacturing technique of a liquid crystal panel using TFTs is used for an area sensor as a solid state image pickup apparatus having photodetecting devices each for converting visible light into an electric signal. By arranging a converting layer for converting an X-ray (radiation) into a visible light beam onto the surface, such an apparatus is also used as a radiation image pickup apparatus. According to such a panel which reads a light irradiation amount, unlike an image display apparatus such as a liquid crystal panel, it is particularly important to accurately transfer charges accumulated in each pixel. However, for example, if a threshold voltage of the TFT is changed due to an external action, an image variation occurs in a fetched image. In the radiation image pickup apparatus, therefore, the following conditions are required of the photodetecting devices and the TFTs.
(1) An amount of light irradiated every device is accurately accumulated as charges.
(2) The charges accumulated in each device are accurately transferred.
Owing to the progress of the recent technique of TFTs for liquid crystal displays, there has been proposed a radiation image pickup apparatus formed by combining: a sensor array constructed by photodetecting devices using amorphous silicon (hereinbelow, abbreviated to a-Si) and switching TFTs; and a phosphor for converting a radiation into visible light or the like. Digitization has also been accomplished in a medical image field. Since a radiation image can be momentarily read out, instantaneously displayed onto a display, and fetched as digital data by such a radiation image pickup apparatus, storage, modification, transfer, and the like of the data can be performed. However, for example, according to a bottom gate type TFT, since source-drain electrodes and a channel portion of the TFT are arranged in the upper portion, there is a feature such that it is influenced by an external action and the threshold voltage changes. Particularly, if the photodetecting device is arranged, for example, in such a form as to cover the TFT in order to improve a numerical aperture, a back channel effect is obtained for the TFT by an influence of electrons or holes which are generated in the photodetecting device and a phenomenon such that the threshold voltages of the TFTs of the pixels are different occurs.
Therefore, for example, in a solid state image pickup apparatus in which a photodetecting device is arranged to an upper portion of a TFT, it is necessary to cover a channel upper portion of the TFT with an electrode.
As a conventional example, according to a proposition of Japanese Patent Application Laid-Open No. 6-216359 by Casio Computer Co., Ltd., a TFT device has a structure in which a source electrode and a drain electrode are sandwiched by a top gate electrode and a bottom gate electrode. In the patent literature 1, since it has the structure in which a semiconductor layer of the TFT device is also used as a photoelectric converting layer, it is difficult to obtain preferable values with respect to both of characteristics such as a switching speed and the like of the TFT and converting efficiency as a photoelectric converting device and there is a relation of tradeoff between them.